1. Field of Invention
The invention relates, in general, to energy transport and in particular to thermochemical energy transport.
2. Description of the Prior Art
A solar thermal power system is one type of system that uses the sun as an inexhaustible heat source. However, solar energy systems are complicated by the low density of energy in sunlight, about 1 kW/m.sup.2, which necessitates large collection fields for industrially significant amounts of power. The energy gathered by the collection field must be transported to a central point where the energy is converted into electricity or used directly in an industrial process. This invention pertains to a thermochemical method of transporting thermal energy over distances of meters to many kilometers by the use of a novel working fluid. Although such an invention is particularly well-adapted to solar energy, it is to be understood that it is not limited to solar applications but is usable whenever thermal energy needs to be transported.
One of the more common approaches to heat transport involves the heating of a liquid to an elevated temperature at the receiver and cooling of the same liquid at the heat delivery end, herein called the delivery. The fluid can be liquid sodium or liquid phase water or almost any liquid. For transport of appreciable amount of energy, the temperature rise must be large. This necessitates the movement of fluids at high temperatures with the attendant problems of thermal expansion, thermal softening of piping and heat loss while the heat is delivered at relatively low temperatures, i.e. low-quality heat.
Alternatively the heat of vaporization of water can be used to absorb 2.3 kJ per gram of steam (atmospheric pressure) in the liquid-to-steam transformation at the receiver end and to deliver the same amount of energy upon condensation at the delivery. This transport process is efficient but is plagued with all the thermal problems of thermal transport by liquids. Furthermore, for energy delivery at temperature above 200.degree. C., steam systems must use heavy high-pressure piping with severe corrosion problems.
Yet another alternative, the one with which this invention is involved, relies upon reversible chemical reactions, particularly with gaseous components. This is called a thermochemical process since heat is tied up in chemically reacting components. The endothermic reaction is supplied from the solar-energy source. The high-energy vapor phase is transported to the heat delivery where the corresponding exothermic reaction liberates the heat to be used. The vapor, the chemical composition of which is changed at each end of the system, is here called the working fluid. Such a system for thermochemical transport of energy, patented by the inventor in U.S. Pat. No. 3,972,183, uses a working fluid of sulfur trioxide, sulfur dioxide and oxygen. Because of that fluid the transport pipes of that system unfortunately are subject to corrosion if not maintained excessively free of moisture.
Another approach uses the general apparatus described in U.S. Pat. No. 3,972,183, but uses a working fluid that relies on methanation for the production of heat. The present inventor in application Ser. No. 147,412, filed on May 6, 1980, now U.S. Pat. No. 4,347,891 granted Sept. 7, 1982 discloses a methanated gas mixture which relies on the reaction CO.sub.2 +CH.sub.4 .revreaction.2CO+2H.sub.2. The working fluid claimed by that application is consistent with the mixture n(CO)+m(CO.sub.2)+CH.sub.4, where n is between 0 and 0.4 and m is 2 and 7. The mixture is hydrogen deficient relative to carbon and oxygen. The working fluid for these reactions can be transported at modest temperatures and pressures. However, methanation reaction at the delivery needs to be operated at a relatively low temperature, reducing the quality of the delivered heat. Furthermore, there are problems with carbon depositing on the heat exchange tubes and the methanation catalyst, thereby decreasing the efficiency of the system.
Chubb in U.S. Pat. No. 3,958,625 teaches a working fluid of CO.sub.2 +CH.sub.4 and relies on the reversible chemical reaction CH.sub.4 +CO.sub.2 .revreaction.2CO+2H.sub.2. For best energy transfer such a system should be operated at very high pressures, above 45 atmospheres. Such pressures require heavy expensive piping. It is also subject to the same difficulties as the present inventor's application Ser. No. 147,412.
Research carried out in Germany by the Kernforschungsanlage Julich (KFA) teaches a gas mixture consistent with y.multidot.H.sub.2 0+CH.sub.4 where y is any value between 1 and 5. Such a mixture is used as the working fluid in the reversible reaction, CH.sub.4 +H.sub.2 0.revreaction.CO+3H.sub.2. However, it delivers heat at a relatively low temperature and generally requires high temperature in the transport lines to prevent steam condensation. The systems described by KFA have condensed out steam before transport of the working fluid, an expensive complication.